The present invention relates to fluid leak detection, and in particular to the leak detection of gases by odor generated by adding odiferous materials to the gases.
With the advent of the fuel cell technology and a drive for clean fuel, hydrogen gas is emerging as a leading candidate for the fuel of choice. In addition to the benefit of being clean burning, hydrogen may be obtained from an abundant, renewable, resource, water.
For hydrogen to become a consumer fuel for automobile and domestic power generation, safety is paramount. Although safe handling and use of hydrogen is well understood, warnings are needed to alert against any leaks. Hydrogen sensors are commercially available but are not considered to be an absolute safeguard against leaks due to their potential for malfunctioning, flow sweeps, etc. Human senses, in particular, the sense of smell, are considered to be the ultimate safeguard against leaks. Since hydrogen is an odorless gas, odorants are preferably incorporated in hydrogen for easy leak detection. A review of the codes, standards, regulations, recommendations, and certifications on the safety of gaseous fuels is addressed in a report, Proc. U.S. DOE Hydrogen Program Rev. (1996), Vol.2, pages 569–604.
Odorization of gases for leak detection is well known in the natural gas and petroleum gas industries. For example, a paper by M. J. Usher (Proc. Int Scho. Hydrocarbon Meas. 73rd, pages 743–48 (1998)) reviews the history, application, compounds, and safety practices in selecting and applying odorants in the natural gas industry. Mixing small quantities of odorants with gases is a substantially universal practice in natural and petroleum gases. For example, a paper by I. Katuran (Proc. Int. Sch. Hydrocarbon Meas., 64th, pages 325–30 (1989)) reports on natural gas odorants, their safety and handling precautions, handling techniques, and methods of adding odorants to gases.
Nearly all of the methods for odorization of natural and petroleum gases consist of metering a certain amount of the odorant into a gas stream to a level where detection can be made by the human sense of smell. Natural gas for public gas supplies typically contains 5–10 mg of sulfur per cubic meter of gas. However, odorants for hydrogen used as an energy source for fuel cells have unique requirements which must be met. This is because most of the commercial odorants used in gas leak detection act as poisons for the catalysts used in hydrogen based fuel cells, most specifically for the PEM (polymer electrolyte membrane or proton exchange membrane) fuel cells. Chemical compounds based on mixtures of acrylic acid and nitrogen compounds have been adopted to achieve a sulfur-free odorization of a gas. See, for example, WO 00/11120 (PCT/EP99/05639) by Haarmann & Reimer GmbH. However, these formulations are either ineffective or do not have general acceptance by users. Also, in the use of natural gas and other petroleum gases for hydrogen generation for fuel cell applications, sulfur free natural or petroleum gases are needed, or else a desulfurization step must be incorporated in the reforming process, which adds further cost to hydrogen generation.
The PEM fuel cells are sulfur intolerant because sulfur compounds poison the noble metal catalysts used in these fuel cells. If sulfur-containing odorants are used, it would be necessary to remove sulfur containing materials, like mercaptan odorants, from the feed gas using materials like zinc oxide. The sulfur containing materials, like thiophenes, cannot be removed by zinc oxide and may require a hydrodesulfurization process, using hydrogen gas, to remove sulfur. This all will add to the cost of the process.
A further complexity for hydrogen fuel comes from the nature of the hydrogen flame propagation. When gases burn in air, their flames propagate upwards with greater ease than they propagate downwards. This is primarily due to the natural convection of hot burnt gases in an upward direction. For petroleum gases, propane and methane, the upward and downward propagating lean limits of combustion are approximately the same. However, for hydrogen, since they differ by a factor of 2.5, the amount of odorant needed for leak detection in hydrogen could be >2.5 times that needed for methane or propane. The higher quantity of the odorant needed for hydrogen odor detection further complicates the sulfur poisoning problems for hydrogen gas used in the PEM fuel cells.
In several other gas applications, particularly when gases are odorless, toxic, or are otherwise harmful, methods of leak detection using odiferous materials are also desirable. The gases included in this category are, for example, nitrogen, carbon monoxide, nitrogen trifluoride, ethylene oxide, carbon tetrafluoride and other perfluoro gases.
Several other issues also have been encountered in the odorization of the natural and petroleum gases. The key ones are (1) hydrocarbon masking the odor of the odiferous materials, (2) adsorption of the odorant on the storage vessel and pipe walls, (3) reaction of the odorants with low molecular weight mercaptans (naturally occurring in the gas), (4) condensation of the odorants in the gas storage vessel and pipes, and (5) physical scrubbing of the mercaptans from the gas with liquids (associated with the natural gas).
Today, approximately twenty-five different blends are used as natural gas odorants. Of these twenty-five blends, seven blends are more prevalent. Almost all of the odorant agents are sulfur compounds, e.g., mercaptans (tetrabutyl mercaptan, isopropyl mercaptan, normal propyl mercaptan, secondary butyl mercaptans, ethyl mercaptans, normal butyl mercaptan, etc.), thiophenes (tetrahydrothiophene), sulfides (dimethyl sulfide, methyl ethyl sulfide), etc.
In addition to the pungent odors of these chemicals, the chemicals used are also expected to have certain other attributes, such as low vapor pressure (high boiling point), low freezing point, low specific gravity so that they are fully dispersed in the gas, and appropriate thermal properties (e.g., they will not freeze at appropriate temperatures and will not cause over odorization in the hot weather). The general quality requirements, as specified for sulfur containing odorants in ISO/DIS 13734, are: (1) a cloud point of less than −30 degrees Celsius, (2) a boiling point of less than 130 degrees Celsius, and (3) evaporation residue of less than 0.2%.
Requirements for odorants further will likely include an odorant concentration high enough to allow detection with a fuel gas concentration of ⅕ the lean limit of combustion. These requirements exist for natural gas (SAE J 1616, NFPA 52-1992) and petroleum gas (NFPA 58-1989).
Natural gases are generally stored, distributed and used at relatively low pressures (50–500 psi). Hydrogen gas on the other hand is stored and transported at very elevated pressures (up to 10,000 psi). At normal operating temperatures, the odorants added to the high pressure gas storage tend to condense at the bottom of the storage vessels. This results in a non-uniform distribution and release of the odorants with the gas, causing a risk of over and under odorization, thereby taking the reliability away from the leak detection method by the human senses.
A traditional method of using odorants is to meter a predetermined amount of the odorant into the gas stream either continuously in the pipeline or on a batch basis in the storage tanks. Electronic odorant injection systems (refer Zeck, DO53017A) have been designed that inject a prescribed amount of the odorants into the gas stream proportional to its flow rate. An electronic odorization system has five duties that it must perform to provide a safe source of natural gas. Combined, these duties insure that enough odorant is injected to make the gas properly detectable to human beings. These duties include:                1) to inject the proper amount of odorant in proportion to the flow of the natural gas;        2) to properly verify system operation;        3) to provide an alarm upon system malfunction;        4) to display information regarding system performance; and        5) to provide chronological records regarding all aspects of the system performance.        
The typical full-featured electronic delivery odorant system should incorporate the following major components:                1) an injection pump;        2) an odorant meter/totalizer; and        3) a system electronics/controller.        
Dispensing of the odorants may be done either in the gas tank or in the gas delivery pipes. Since the odorants are added to the bulk gas, their concentration varies throughout the tank. The odorant concentration in the delivered gas also is dependent on the speed at which the gas is dispensed. At low dispensing speeds, odorants get adsorbed on the delivery pipe walls, resulting in low odorant concentration in the bulk gas.
Whereas adding odorants in the bulk gas is a simple method, it requires the whole gas stream to be contaminated and reasonably large quantities of the odorants have to be used. Odorants tend to condense in high pressure and low temperature storage and phase separate from the gas, thereby causing a gradient of the odorants in the gas.
Thus, the whole process of dispensing odorants to the gas and maintaining a uniform concentration of odorant in the gas is complex and requires major improvements.
It is, therefore, desired to have the use of odorants in hydrogen gas storage and delivery systems in which the odorants are released in the gas in such a manner that a uniform quantity of the odorants is maintained all of the time.
It is further desired to have the use of odorants in hydrogen gas storage and delivery systems in which the odorants are distributed in the bulk gas in such a way that it maintains an almost constant concentration of the odorant in it throughout the supply of the gas.
It is still further desired to have such a system and method which overcome the difficulties and disadvantages of the prior art to provide better and more advantageous results.